CDMA systems use soft handoffs as one means of reducing interference. A “handoff” occurs when a mobile station moves between cells or sectors. In a traditional “hard” handoff, the connection to the current radio base station (RBS) the source base station) is broken and a connection is made with the new RBS (the target base station) to resume communication with the mobile station. This is known as a “break before make” handoff. Because all cells or sectors in a CDMA system use the same frequency, it is possible to make the connection to the target RBS before terminating the connection with the source RBS. This is known as a “make before break” or “soft” handoff. A soft handoff requires less power, which reduces interference and increases system capacity. A soft handoff is also more reliable (i.e., less dropped calls) because the new connection is made before the old connection is broken.
During operation, the mobile station measures the received signal strength of pilot signals, or pilots, from nearby cells or sectors and reports its measurements to the network in a pilot strength measurement report. A base station controller determines what pilots to include in the active set of the mobile station based on the pilot strength measurements. A soft handoff occurs when the active set of a mobile station contains two or more pilots. During a soft handoff, a mobile station may transmit and receive from two or more radio base stations.
During normal operations in a typical wireless communication network, the mobile station will be in soft handoff approximately 50 to 60% of the time. When the mobile station is not in soft handoff, the active set comprises the pilot for a single cell or sector. Both the RBS and the mobile station monitor the performance of the communication channel and can request handoffs. When a handoff is requested, the BSC adds pilots to the mobile stations' active set. As the pilot strength from a given RBS in the active set falls below a defined point as measured by the mobile station, that RBS is removed from the mobile station's active set.
There are circumstances when communications with an RBS in the active set yields poor performance even though the pilot strength measurements indicate that channel conditions are good. Such circumstances may arise, for example, when the RBSs selected based on pilot strength measurements are very heavily loaded.